Specific anthracycline binding proteins were identified in mouse liver by photoaffinity labeling with a photoactivatable daunorubicin analogue. The influence of these cytoplasmic receptors on subcellular disposition, metabolism and antineoplastic activity was studied. The metabolism and disposition of marcellomycin in mice was examined. The presence of a previously unrecognized enzyme accounting for a major biotransformation to non-fluorescent metabolite(s) was identified.